1. Field of the Invention
This invention is related to an image processing apparatus. More specifically, this invention is related to an image adjusting apparatus for dynamically adjusting the contrast of image signals.
2. Description of the Prior Art
Generally, the contrast of a natural image is related to the brightness ratio between light areas and dark areas in the image. Although the increase of contrast might result in slight distortion for an image, a proper increase in the contrast provides more comfortable visual feelings for most people. Hence, how to enhance the contrast of images is an important issue of image processing.
In prior arts, the contrasts of images can be enhanced by adjusting the Gamma parameter of monitors, that is to say, the contrasts are adjusted by changing the relation between gray scale levels and brightness of the monitor. Most monitors provide several Gamma parameters or a function of on screen display (OSD) for users to adjust contrasts. But a constant contrast setting cannot fit every image. For example, contrasts of images with higher brightness can be increased by setting a larger Gamma value. But applying a larger Gamma value on an image with lower brightness would worsen the quality of the image by making the areas of low brightness too dark and losing details. Therefore, better contrast adjustments must be dynamic according to different images, that is, adjustments should be made according to the brightness of individual image.
The idea of enhancing contrasts is to make the bright part of an image brighter and make the darker part darker. At the same time, the colors of images should not be affected. It is better to adjust contrasts in Y/C color spaces than in traditional RGB color spaces because each RGB signals respectively includes a brightness component and a color component and the brightness component which relates to contrasts cannot be individually adjusted unless the two components are separated. On the contrast, brightness and colors are separated as YPbPr signals or YCbCr signals in some high quality image signals. In prior arts, some monitors use video decoders or scaler chips to change the Y vector of YPbPr signals or YCbCr signals, such that the contrast of an image is adjusted. In prior arts, most methods for adjusting contrasts use YCbCr signals or YPbPr signals. However, as for the quality of separating brightness and colors, YCbCr and YPbPr signals are worse than CIE Lab signals.
Accordingly, this invention provides a dynamic image adjusting apparatus to overcome aforementioned problems. Furthermore, the color space used in this invention is the CIE Lab color space proposed by CIE (Commission Internationale de L'Éclairage).